1. Field of the Invention
The present invention relates to a method for reducing the temperature dependence of a capacitive sensor.
The invention also relates to a capacitive sensor construction.
2. Discussion of the Background Art
Capacitive sensors are used in the measurement of, for instance, acceleration, angular acceleration, angular velocity, and pressure.
According to the state of the art, in capacitive sensors, an insulating layer is used to connect the mechanical construction from the support areas of the sensor and to create a capacitor construction. The support area can act as a mechanical support, an electrical insulation, and as part of a gas-tight chamber. The support area is often located at the edges of the sensor.
The insulation can be a unified glass disc, on the inside of which a electrode is formed by means of, for example, thin-membrane metallizing.
A construction of this kind causes a large state of stress in the silicon disc contained in the moving mechanical component, due to the difference in the thermal expansion coefficients of the materials. This, in turn, causes structural deformations as the temperature changes and consequently a temperature dependence in the electrical properties.
The insulating layer can be manufactured on top of the silicon layer by joining a silicon disc to a glass disc, or, for example, by melting glass onto the surface of the silicon. The silicon under the glass surface reduces the structural deformations and stresses and also permits the silicon structure forming the support to be used as an electrical terminal.
The invention is intended to create an entirely new type of method and sensor construction, by means of which the drawbacks of the state of the art described above can be eliminated.
The invention is based on forming the insulating layer to be thinner, at least in the active areas of the sensor, than in the support areas of the sensor. The term active areas of the sensor refers to the areas affecting the changing capacitance of the sensor. Typically, this insulating area is of glass, silicon oxide, or some other insulating material generally used in microelectronics.
Considerable advantages are gained with the aid of the invention.
In a capacitive sensor, it is possible to combine the requirements of a small parasitic capacitance and a small deformation (deflection) of the disc. The thermal coefficient of the capacitance can thus be made small over a specific temperature range.